1. Field of the Invention
The present invention relates to a photomask and a pattern forming method.
2. Description of the Related Art
With the recent progress in miniaturization of semiconductor devices, the semiconductor devices with 50 to 150 nm design rules have been developed. A photolithography process in the manufacture of a semiconductor device uses light in the near-ultraviolet or ultraviolet region as an exposure light source, e.g., G-rays (wavelength: 436 nm), i-rays (wavelength: 365 nm), a KrF excimer laser (wavelength: 248 nm), an ArF excimer laser (wavelength: 193 nm), or an F2 laser (wavelength: 156 nm). At present, a KrF excimer laser beam or ArF excimer laser beam is used to form a resist pattern having a smaller line width on a semiconductor substrate.
An exposure technique that uses such high-energy light with off-axis illumination, a phase shift mask, etc. is generally known as a super-resolution technique. The off-axis illumination can be incorporated easily into a conventional exposure apparatus and provide a high degree of freedom in the pattern layout design of a circuit. Therefore, it is often used for transferring a pattern with a line width of 50 to 150 nm.
When the line width of a resist pattern formed by transfer is about one-half the wavelength of exposure light in the super-resolution technique, it is difficult to ensure sufficient resolution and exposure margin by so-called ½ annular illumination. Thus, ⅔ annular illumination or quadrupole illumination has been employed recently.
However, the super-resolution techniques such as ⅔ annular illumination and quadrupole illumination have the following problem. For example, when a mask including a mask pattern of two line patterns 20, 21 intersecting to form a T-shape (FIG. 9) is used and the mask pattern is transferred onto a semiconductor substrate, constrictions 22 are generated in part of a resist pattern (FIG. 10). These constrictions 22 may cause disconnection during manufacture of a semiconductor device.
JP 5(1993)-67550 A discloses a technique to solve the similar problems. When a large pattern branches out into fine line patterns, a resist pattern corresponding to the fine line pattern tends to be constricted in the branch portion. To eliminate the constriction, this technique uses a mask in which a small pattern of resolution limit or less is arranged on both sides of a submicron width mask pattern at the position 0.4 to 0.75 λ/(NA·K) away from the junction between wirings. Here, λ is the wavelength of exposure light, NA is the numerical aperture of a lens used, and K is a lens reduction ratio.
However, this conventional technique is not available for the transfer of a fine circuit pattern having a line width of 50 to 150 nm by the super-resolution technique. Although there is a similarity of the phenomena of constriction to be generated in part of a resist pattern, the mechanism of the constriction is different due to, e.g., a difference in exposure illumination, as will be described later. Therefore, the technique disclosed in JP 5(1993)-67550 A cannot eliminate the constrictions 22 shown in FIG. 10.